1. Field of the Invention
The present invention relates to a power source apparatus for a display for generating and supplying a predetermined voltage to each section, and an image display apparatus incorporating the same (e.g., a liquid crystal display apparatus and the like).
2. Description of the Related Art
Conventionally, a liquid crystal display apparatus is provided with a display panel comprising a display section. The display section has a plurality of pixels arranged in a matrix. Each pixel is provided with a thin film transistor (TFT). A display signal is applied between the pixel electrode and the common electrode (counter electrode) of each pixel to perform image displaying. Typically, the TFT is formed of a MOSFET having a source electrode, a drain electrode and a gate electrode. The drain electrode of the TFT is connected to the pixel electrode of the pixel. The source electrode of the TFT is connected to a source bus line (source line) on which a display signal is transferred. The gate electrode of the TFT is connected to a gate bus line (gate line) on which a TFT drive voltage is transferred.
FIG. 8 is a block diagram showing a configuration of a conventional liquid crystal display apparatus.
Referring to FIG. 8, a liquid crystal display apparatus 100 comprises a display controller 110, a power source circuit 120 (power source apparatus for a display), and a display panel 130 having a display section 130a. 
The display controller 110 receives I/O (Input/Output) signals output from an external system controller 200 and outputs various signals, such as display data (display signals), to the display panel 130.
The power source circuit 120 outputs a source reference voltage to the source electrode (pixel electrode) of the TFT of each pixel in the display panel 130 through a corresponding output terminal thereof. The power source circuit 120 also outputs a common reference voltage to the common electrode of the TFT of each pixel and outputs a gate HIGH voltage and a gate LOW voltage to the gate electrode of the TFT.
The display panel 130 further comprises a gate driver 130b for driving a plurality of gate lines GL and a source driver 130c for driving a plurality of source lines SL. In the display section 130a, a plurality of pixels are arranged in a matrix such that each pixel is located in the vicinity of the intersection of the gate line GL and the source line SL and the pixel is connected via a TFT to the gate line GL and the source line SL. The display panel 130 receives various signals (e.g., display data and the like) output from the display controller 110 and the above-described predetermined output voltage output from the power source circuit 120, and performs image displaying on the display section 130a via the gate driver 130b and the source driver 130c. 
FIG. 9 is a timing chart of signal voltages applied to the display panel of the liquid crystal display apparatus of FIG. 8.
A pixel voltage, a common voltage and a source voltage as shown in FIG. 9 are applied to each pixel. The pixel voltage is a voltage synthesized based on the difference between the source voltage and the common voltage, which is an alternating voltage having a pulse form. A gate voltage is applied to select pixels on a line (Nth line; N is a natural number) in the display panel 130 at predetermined time intervals.
The source/common reference voltage, the gate HIGH voltage and the gate LOW voltage are constant whenever applied to the display panel 130 for driving.
In the liquid crystal display apparatus 100 of FIG. 8, electric charges often remain in the pixel electrode (and the common electrode) of each pixel in the display panel 130 even after the source/common reference voltage, the gate HIGH voltage and the gate LOW voltage of the power source circuit 120 are turned OFF, as shown with arrow A in FIG. 9. The electric charges cannot be erased in a short time. Therefore, it is likely that an image displayed on the display section 130a of the liquid crystal display apparatus 100 persists after turning OFF the power source (such a persisting image is herein referred to as an afterimage).
The afterimage occurring on the display screen of the display section 130a in the display panel 130 will be described with reference to FIGS. 10A and 10B. FIG. 10A shows the fall and rise of the pixel voltage immediately after the source/common reference voltage, the gate HIGH voltage and the gate LOW voltage of the power source circuit 120 are turned OFF. FIG. 10B shows an afterimage on the display section 130a of the display panel 130 in association with the pixel voltage of FIG. 10A.
As shown in FIG. 10A, the fall and rise of the source/common reference voltage supplied to the display panel 130 are gradually transitioned. Therefore, an afterimage occurs as show in FIG. 10B during a time for which electric charges are not sufficiently removed from pixels.
In the case of applications where the liquid crystal display apparatus 100 is employed in the display section of a portable apparatus, such as a mobile telephone or the like, a battery is used to drive the apparatus and low power consumption is thus required. For this reason, the liquid crystal display apparatus 100 has to be driven using a low frequency. In this case, the pixels of the display panel 130 in the liquid crystal display apparatus 100 are designed to have a high level of ability to retain electric charges in order to display images using display signals. This ability makes the above-described afterimage problem more noticeable.
In order to solve the afterimage problem, for example, a discharge circuit for removing unnecessary electric charges has been reported (see FIG. 11, for example).
In a discharge circuit shown in FIG. 11, a power source circuit 120 includes a booster circuit 140 which generates a source/common reference voltage, a gate HIGH voltage and a gate LOW voltage. These voltages are output as output voltages from the power source circuit 120 to a display panel 130. An output line is connected between an output terminal of the booster circuit 140 and an input terminal of the display panel 130. A discharge resistor R and a capacitor C are connected in parallel between the output line and GND (the earth). The booster circuit 140 generates a predetermined source/common reference voltage, gate HIGH voltage or gate LOW voltage based on an externally input voltage.
The above-described discharge circuit (a parallel circuit of the discharge resistor R and the capacitor C) discharges unnecessary electric charges remaining in each pixel in the display panel 130 to GND (the earth) when the source/common reference voltage, the gate HIGH voltage and the gate LOW voltage are in the OFF state in the power source circuit 120. Thereby, the afterimage on the display screen can be prevented.
Japanese Laid-Open Publication No. 61-162029 discloses a liquid crystal drive circuit (see FIG. 13), in which in order to prevent display abnormality due to the gradual decrease of the waveform of a voltage applied to a display panel LCD after turning OFF the power source, a circuit 200 is provided for extinguishing the voltage applied to the display panel LCD before the voltage of the power source line starts decreasing. In this liquid crystal drive circuit, a direct current power source DC is connected via a diode D and a power source switch SW to a power source terminal A of a liquid crystal driver DR, and a capacitor C is connected between the power source terminal A of the liquid crystal driver DR and the earth GND. When the power source switch SW is opened to interrupt the connection between the direct current power source DC and the liquid crystal driver DR, a voltage drop at the power source terminal A of the liquid crystal driver DR is delayed due to discharge of the capacitor C. This is because a current is prevented by the diode D from flowing from the capacitor C to the terminal A′. Therefore, the signal voltage of the signal terminal A′ drops earlier than the voltage of the power source terminal A. Therefore, the voltage applied to the display panel LCD becomes 0 V before the voltage drop of the power source line connected to the power source terminal A of the liquid crystal driver DR.
Japanese Laid-Open Publication No. 6-160806 discloses another liquid crystal display apparatus. When a power source switch is turned ON or off, streak display defects appear on the screen. To avoid this problem, the liquid crystal display apparatus is provided with a scanning continuation circuit. A scanning electrode drive circuit is operated by the scanning continuation circuit to continue the scanning of scanning pulses after the output of an operational power source voltage is terminated and until a scanning pulse voltage decreases below an effective display threshold voltage of a liquid crystal layer. Thus, by continuing the scanning of scanning pulses after terminating the operational voltage power source, lower direct current voltage components remain, thereby making it possible to prevent appearance of streak display defects.
In the conventional configuration shown in FIG. 11, the resistance of the discharge resistor R is set to a low value so that unnecessary electric charges remaining in each pixel of the display panel 130 can be sufficiently quickly discharged to GND (the earth), i.e., the fall of the power source is caused to be steep. In this case, for example, a current of about 0.1 mA consistently flows through the discharge resistor R in driving, so that the total power consumption of the liquid crystal display apparatus 100 is increased by about 1.0 mW. Low power consumption cannot be achieved. Thus, an attempt to overcome the afterimage problem by steepening the fall of the power source unfortunately leads to an increase in power consumption. If the resistance of the discharge resistor R is set to be relatively high in favor of power consumption, the fall and rise of the power source are moderate as indicated by arrow B in FIG. 12. In this case, electric charges are not sufficiently removed from pixels, resulting in an afterimage.
A latch-up phenomenon or the like occurs depending on the discharge conditions for a pixel, which may destroy a driver IC for driving liquid crystal provided in the display panel 130. To address the latch-up phenomenon or the like, a diode is provided in an output portion of the liquid crystal driver IC, however it is insufficient. Specifically, when a main power source falls, the voltage becomes unstable, leading to destruction of the display driver.
When unnecessary electric charges remaining in pixels in the display panel 130 are only discharged to GND (the earth) by the discharge circuit of FIG. 11, the pixel is affected by crosstalk when discharging from the output line. To address this crosstalk problem, unnecessary electric charges remaining in the pixel electrode are discharged to GND (the earth) by sensing the OFF state (fall) of the main power source and applying a HIGH voltage to the gate electrode of the TFT of the pixel as indicated by arrow C in FIG. 12. The discharge from the pixel electrode depends on the final state of display (display image) immediately before turning OFF the power source. As indicated by arrow D in FIG. 12, the HIGH voltage period is unstable due to the power source in the off state. Therefore, the period of time for discharging from the pixel (electric charge removing period) cannot be adjusted. Thus, similar to the portion indicated by arrow A in FIG. 9, an afterimage is likely to occur.
Specifically, as shown in the timing chart (FIG. 12) of signal voltages applied to the display panel 130 of FIG. 11, the fall and rise of the pixel voltage at the plus (+) side and minus (−) side thereof depend on the final state of image displaying immediately before turning OFF the power source when discharging electric charges remaining in pixels. The period of time during which a HIGH voltage is applied to the gate electrode of a TFT is not constant (HIGH period instability). Therefore, the discharging period of electric charges remaining in pixels cannot be adjusted, so that the afterimage problem cannot be completely overcome. Thus, pixel electric charges on the display screen are not uniformly removed, resulting in an afterimage. Since there is a parasitic capacitance between each pixel and the power source circuit 120, the voltage quickly falls, resulting in an adverse effect on the displayed images (crosstalk).
Further, in the case of a small-size liquid crystal display (small-size liquid crystal module) used for a small-size portable apparatus, such as a mobile telephone or the like, the main power source is in the ON state even when the output is in the OFF state (waiting for a call). Therefore, an analog voltage is likely to be applied to a source bus line, resulting in a reduction in the reliability of the liquid crystal display.
In the above-described publications, the display abnormality occurring when the power source is in the OFF state is prevented. However, the above-described problems are not solved therein. As shown in FIG. 14, the discharge of the pixel voltage depends on an image displayed immediately before turning OFF the power source. The electric charge removing period (HIGH period) is unstable. The latch-up phenomenon is also likely to occur. The fall of the power source is gradual. Therefore, electric charges tend to remain in pixels, resulting in an afterimage after turning OFF the power source.